Polyamide resins are generally excellent in rigidity, abrasion resistance, chemical resistance, thermal resistance, and electrical characteristics and have therefore been widely employed as engineering plastics.
Polyamide resins, however, still leave room for improvements in impact resistance, molding stability and the like, which have constituted hindrances in exploiting their utility. Thus, various proposals have hitherto been made to improve impact resistance of polyamide resins.
Typical proposals include a reinforced multi-phase thermoplastic resin composition comprising a polyamide having dispersed therein an ethylene copolymer, e.g., an ethylenemaleic anhydride-acrylic ester copolymer, to a particle size of from 0.01 to 1.0 .mu.m as disclosed in JP-B-55-44108 (the term "JP-B" as used herein means an "examined Japanese patent publication"); a high-impact polyamide comprising a polyamide having dispersed therein a maleic anhydride-modified ethylene-propylene rubber to a particle size of not more than 0.36 .mu.m as disclosed in JP-A-60-221453 (the term "JP-A" a used herein means an "unexamined published Japanese patent application"); and a high-impact polyamide resin composition comprising a polyamide having dispersed therein an ionomer resin to a particle size of from 0.005 to 3 .mu.m as disclosed in JP-A-58-108251.
It is well known, as stated above, that properties of a polyamide resin composition greatly depend on the dispersed state, i.e., the micro-phase separation structure, of the constituent polymer. It is hence important to elaborate on the optimum phase separation structure for manifestation of excellent physical properties. Most of the so far proposed impact resistant polyamide resins have revealed through microscopic observation a simple two-phase separation structure (particle structure) called sea-island structure in which a second component having rubbery properties is finely dispersed as a particulate phase (island phase) in a polyamide matrix phase (sea phase).
The latest report, POLYMER COMMUNICATIONS, Vol. 29, pp. 163 (1988), describes that polyamide 6.6 and 20% by weight a maleic anhydride-modified ethylene-propylene rubber are melt-kneaded in a 30 mm.phi. twin-screw extruder to provide a multi-phase structure. According to the examination by the present inventors, however, since the multi-phase structure in this report is unstable, if the compound is further melt-kneaded or a part or the whole of the sprue and runner is recycled to injection molding, etc, it has turned out that the multi-phase structure becomes a sea-island two-phase structure and, also, various physical properties are deteriorated.
It is also known that polyamide resins having a higher terminal amino group concentration give better results as disclosed in JP-B-62-25182, suggesting use of a polyamide resin having a concentration ratio of terminal amino group to terminal carboxyl group of 3.5 or more and in JP-A-59-164359, teaching use of a polyamide resin having a terminal amino group concentration of 5.5.times.10.sup.-5 eq./g or more. Further, JP-B-61-37305 proposes use of a polyamide resin having a relative viscosity of at least 3.5.
It has also been disclosed in JP-A-63-199755 and JP-A-63-235365 that a combined use of an ethylene-(meth)acrylic ester-maleic anhydride copolymer with a polyfunctional compound as a partial crosslinking agent provides a polyamide resin composition exhibiting fairly improved impact resistance over the conventional polyamide resins, but the composition is still insufficient in maintaining physical properties, such as thermal resistance, rigidity, impact resistance, and processability, in a good balance. That is, the improved impact resistance and softness are offset by other mechanical properties poorer than those of a polyamide resin per se, such as rigidity, tensile strength, hardness, thermal resistance, and processability.
It has turned out that the conventional sea-island structure has its own limit in making a balance between impact resistance and rigidity of a polyamide resin which are conflicting with each other while satisfying molding processability, i.e., high fluidity. In other words, a specific phase separation structure should be established before a material exhibiting impact resistance combined with rigidity, thermal resistance, and moldability can be obtained. None of the above-described conventional techniques is based on this point of view.